Manufacture of plaster of paris mold having sprayed metal oxide linings and product

ABSTRACT

A metal or ceramic form of desired shape and conditioned to permit its later removal from a flame sprayed layer is flame sprayed to form a metal oxide layer of desired thickness having a porosity of 10 percent or more. A plaster of paris slurry is then poured around this layer and allowed to set up at which time the form is removed leaving a plaster of parismold with a flame sprayed metal oxide wear resistant working lining suitable for use in slip casting, etc.

United States Patent Teague 1 June 27, 1972 s41 MANUFACTURE OF PLASTEROF 2,990,601 7/1961 Wagner ..264/80 PARIS MOLD HAVING SPRAYED 3,228,650l/1966 Gilliland et al. .....264/225 METAL OXIDE LININGS AND 3,405,21210/1968 Fraser et al ..264/226 PRODUCT 3,536,800 10/1970 Hubbard..264/338 2,984,887 5/1961 Thiess ..264/338 [72] Inventor: ErnestDouglas 'lgague, welw n Ga -d 2,879,196 3/1959 Brucker ..264/3l7 EnglandFOREIGN PATENTS 0R APPLICATIONS A N [73] 23: f 'gfgi welwy" 1,243,8829/1960 France ..264l86 [22] Filed: Feb. 17, 1970 OTHER PUBLICATIONS [21]Appl. No.: 12,120 J. Oliver et al., Metal Spraying of Mouldmakers Cases,

December, 1965, Ceramics at 20- 22 and 24. Related U.S-. ApplicationData J. A. Carter, Permeable Mold Materials," March, 1966, [63]Continuation-impart of Ser. No. 633,129, April 24, Cmmw 263- 265 1967abandoned' Primary Examiner-Philip E. Anderson Assistant Examiner-JohnH. Miller [30] Forelgn Application Priority Dam Attorney-Stevens, Davis,Miller & Mosher April 29, 1966 Great Britain ..l8,986/66 [57] ABSTRACT[52] A metal or ceramic form of desired shape and conditioned to [51]Int Cl C04, 39/08 332i) 13/04 7/36 permit its later removal from a flamesprayed layer is flame [58] Field Xvi/56 60 62 2 87 220 sprayed to fonna metal oxide layer of desired thickness having a porosity of 10 percentor more. A plaster of paris slurry is 264/225 249/114 then poured aroundthis layer and allowed to set up at which [56] Rekrences Cited time theform is removed leaving a plaster of parismold with a flame sprayedmetal oxide wear resistant working lining suitable for use in slipcasting, etc.

9 Claims, 2 Drawing Figures MANUFACTURE OF PLASTER F PARIS MOLD HAVINGSPRAYED METAL OXIDE LININGS AND PRODUCT This application is acontinuation-in-part of my copending application Ser. No. 633,129, filedApr. 24, 1967 and now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates to moldsfor casting and to a method of forming a mold with a refractory anderosion resistant coatmg.

According to the present invention there is provided a method of forminga mold body with a refractory coating comprising flame spraying arefractory coating on a former, forming the body of the mold on the sideof the coating opposite to the former and removing the former from thecoating. The body of the mold may be of plaster of Paris or may be acast refractory material.

A method of flame spraying for use in the method according to thepresent invention is exemplified by U.S. Pat. No. 2,707,691, toWheildon, Jr., which discloses suitable flame spraying techniques. Amongother methods which should be mentioned as being suitable for layingdown the refractory coating via flame spraying, the art of oxyacetyleneor plasma gun spraying is specifically included within the meaning offlame spraying of the present invention. According to the method ofplasma gun or electric arc spraying, a metal oxide is used in powderform, the metal oxide particles being rendered molten in the arc andsubsequently conveyed onto a surface to be coated by a stream of hotgas.

To aid in the removal of the former from the coating, the surface of theformer is preferably blasted with very fine abrasive before applicationof the refractory coating; this surface of the former may also be coatedwith a release material before the coating is applied.

The free surface of the coating supported on the former is usuallyrough, so that the mold body built up on this rough'surface adheresstrongly to the coating and prevents removal of the coating with theformer when the composite coating and body structure is removed from theformer.

When the refractory coating has a porosity of about percent, thecomposite mold with the refractory coating and adherent body may be usedfor the casting of pottery ware. The provision of such a refractorycoating of the desired porosity of about 10 percent is provided by theuse of a flame sprayed metal oxide such as a refractory coating of fusedalumina. In addition to alumina, other metal oxides which provide arefractory coating with a porosity of about 10 percent are alsocontemplated within the scope of the present invention. As furtherexamples of suitable metal oxides there may be mentioned zirconium oxideand other oxides listed in U. S. Pat. No. 2,707,691 which provides aporosity of about 10 percent.

Molds which have been made from plaster of Paris are known in the priorart, and have many advantages, including the accurate reproduction ofthe surface of a master form by casting a fluid plaster of Parismixture. The ease of machining of the reverse side of the dried mold toany shape required also renders plaster of Paris molds highly desirable.The high degree of porosity and capillacity of the dried mold isparticularly advantageous, as it readily absorbs water from plastic claybeing formed in the mold and permits vacuum or negative pressure to beapplied to the reverse side of the mold to facilitate withdrawal ofwater, particularly when high production rates are required.

Plaster of Paris molds have, however, relatively little resistance toabrasion, erosion or scratching, particularly when wet. Damaged moldscan result in defective pottery ware which are expensive to rectify inthe pottery moldings or by replacement of the mold form.

The problem of resistance to abrasion, erosion and scratching seriouslydetracts from the value of making molds of plaster of Paris.Furthermore, the prior art has not provided plaster of Paris molds whichare coated with a metallic coating resistant to abrasion, because tomake such a coating would negate the reason for using plaster of Parisfor the mold, i.e., the water withdrawing property of plaster of Pariscould not be used to draw water from the plastic clay being formed inthe mold due to the solid metallic coating. Thus, notwithstanding thefact that the art has long been aware of the problems of abrasion,erosion and scratching attendant to plaster of Paris molds, asatisfactory method of solving these problems without destroying thebeneficial properties of the plaster of Paris has not been found priorto the present invention.

An object of the present invention therefore is to provide a solution tothe problem of abrasion, erosion and scratching of plaster of Parismolds without destroying the beneficial water withdrawing capacity ofthe mold. Thus, according to the present invention, a mold having acontinuous surface of fused metal oxide, particularly alumina, with aporosity of about 10 percent is provided. This mold can be accuratelyformed and has a porous surface, which surface is resistant to abrasion,erosion and scratching. Such a mold having a plaster of Paris mold bodyshows the surprising and unexpected result that the flame sprayedcoating does not restrict or inhibit the very strong water withdrawalproperties of the plaster of Paris.

It has been found that it is not readily possible to apply refractorymaterials by flame spraying directly to the plaster of Paris andfurthermore, flame sprayed coatings usually have a rough free surfacewhich would require difficult and expensive grinding to provide thenecessary accurate and smooth surface for the mold. By suing thetransfer coating technique outlined above, the rough surface of thecoating opposite to the side deposited on the former, is used to providestrong adhesion with the plaster of Paris mold body, whereas the side ofthe coating that is deposited directly on the former has a smoothnessthat substantially duplicates the smooth surface of the former on whichthe coating is first applied.

Suitable materials for the former include most metals, porcelain andother high temperature stabilized ceramic materials. Low adhesionbetween the coating and the former results when the surface of theformer is given a very fine abrasive blasting treatment before thecoating is sprayed thereon, for example, the surface of the former maybe blasted with a 220 mesh fused alumina abrasive. A release coatingsuch as a silicone resin or polytetrafluoroethylene dispersion may thenbe applied by an aerosol to the blasted surface.

After the flame sprayed coating has been deposited on the fonner, themold body is then cast on the free surface of the coating, the castingmaterial forming the body, penetrating the interstices of the roughsurface to provide a strong adhesion between the coating and the castmold body. When the former is removed, the coating is retained on thecast mold body and remains an integral part thereof, to provide a moldwith an exposed surface accurately reproducing the surface of theoriginal former. The body of the mold may be cast from plaster of Parisor castable refractory cements.

Molds formed from plaster of Paris may be used in slip casting, in whichceramic bodies are formed by placing a relatively thin cream with a highwater content in the mold. The mold, being porous, absorbs the watercausing a layer of high solid content material to build-up on the walls.Excess slip is poured away, and the mold with its formed layer is placedin a slow drying oven. As the cast material dries, it shrinks slightlyand releases itself from the mold walls and can easily be removed fromthe mold for firing.

The thickness of the refractory coatings sprayed on the former may be asgreat as desired, but the preferred range is between 0.002 inches and0.005 inches. A preferred embodiment of this invention comprises athickness of about 0.004 inches.

An example of the invention related to pottery bodies such as plateswill now be described with reference to the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic cross-sectionthrough an arrange ment for forming a mold body with a flame sprayedporous coating on one side; and

FIG. 2 is a diagrammatic cross-section through an arrangement forforming a plate using the mold illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT As illustrated in FIG. 1, asmooth-surface steel master form 1 1 that has been grit blasted andcoated with a release agent, has a refractory aluminum oxide coating 120.004 inches thick flame sprayed on its smooth surface, making use ofthe flame spraying system shown in U. S. Pat. No. 2,707,691. The coating12 has about percent open and interconnected pores. The master form 1 1is surrounded by a band or steel ring 13 to support the sides of a fluidmaterial poured on top of the coating 12. The fluid material is plasterof Paris having the consistency of thin cream. The fluid material flowsonto the rough side of the flame sprayed coating opposite to the smoothside deposited on the form and the fluid material is supported by theband or ring walls 13 until the plaster of Paris hardens and is set. Theupper surface of the plaster of Paris filling 14 sets flat, and issubsequently machined to the dashed line contour 15 to provide a porousmold body 21 which is approximately 3 inches thick.

The machined mold 21 is removed from the master form 11 and is thenready for use. For such purpose mold 21 is mounted in the slip castingarrangement of FIG. 2, with the refractory coating 12 uppermost.

The material 22 to be cast or formed into the shape of a plate, as hereshown, is worked into a wet plastic condition and a measured quantity isplaced on top of the smooth surface of the coating 12 and shaped on itsupper side by a rotating shaper 23. The liquid from the material 22 iswithdrawn through the porous surface 12 into the plaster of Paris moldbody to be elminiated from the material 22, which may then be removedfrom the mold. Due to the hard wear resistant characteristic of coating12, the conventional slip casting operation may be repeated innumerabletimes without undue wear showing on the mold surface.

The mold functions to permit water to pass from the material 22 throughthe interconnecting pores of coating 12 and into the plaster of Paris.The latter material has a very large capacity to extract water from aslip, that may be assisted by vacuum equipment if the process is to bespeeded up. The mold constructed as above described thus incorporatesthe basic plaster of Paris mold body in a structure having a smooth slipreceiving surface that produces the desired finish on the slip castobject and yet the surface of the mold is in no way affected by repeatedrecycling of the slip casting step so that precision slip cast objectscan be repeatedly made in the same mold.

What is claimed is:

l. A method of forming a composite porous slip casting mold structurehaving a smooth, hard and wear resistant slip receiving surface and aporous water absorbing body to effect removal of the liquid phase of theslip to produce a finished slip casting comprising spraying moltenparticles of a refractory metal oxide material against the smoothsurface of a metal or ceramic form conditioned to permit the form to belater separated from the flame sprayed material and having the shape ofthe desired casting, freezing the particles in situ on the form toproduce a hard porous layer thereon, said hard layer having a smoothsurface on one side that is formed by reason of the molten particlessolidifying while in contact with the smooth surface of the form and theexposed surface of the solidified layer on the form being relativelyrougher, surrounding the rougher surface of the layer on the form withan activated mass of plaster of Paris material having the consistency ofthin cream, allowing the plaster of Paris material to set and harden tobond it to the rougher surface of the layer, and then removing thecomposite structure including the layer of metal oxide and hardenedplaster of Paris from the form whereby to provide a porous slip castmold structure adapted to rapidly absorb water from a slip placed in it.

2. A method as in claim 1 wherein the metal oxide is an alumina materialand is sprayed to have a porosity of about 10 percent.

3. A method as in claim I wherein the hardned plaster of Paris mold isshaped subsequent to setting.

4. A method as in claim 1 wherein the metal oxide layer is sprayed to beapproximately in the range of 0.002 to 0.005 inches thick.

5. A composite slip cast mold structure having a smooth porous surfacein the mold to receive the slip and a porous body to absorb the waterfrom the slip comprising a flame sprayed porous refractory metal oxidelayer that is initially formed to shape by being sprayed as moltendroplets and frozen in situ on a metal or ceramic form conditioned topermit the form to be later separated from the flame sprayed materialand having the shape of the slip casting to be produced and a mass ofplaster of Paris material integrally bonded to said solidified layer;said metal oxide layer having a smooth exposed slip receiving surfaceand a rougher interface surface that is bonded to said plaster of Parismaterial; said smooth surface having approximately the same degree offinish as the surface of the mold onto which the molten metal oxide wassprayed; said rougher interface surface being the exposed surface of thelayer resulting from flame spraying and freen'ng the metal oxide in situon a mold; and said metal oxide layer being hard, wear resistant andporous.

6. A mold structure as in claim 5 wherein the slip receiving layer isalumina.

7. a mold structure as in claim 5 wherein the slip receiving layer isapproximately 0.002 to 0.005 inches thick.

8. A mold structure as in claim 6 wherein the alumina layer is about 10percent open and interconnected pores.

9. A mold structure as in claim 8 wherein the porous alumina layer isapproximately 0.004 inches thick.

2. A method as in claim 1 wherein the metal oxide is an alumina material and is sprayed to have a porosity of about 10 percent.
 3. A method as in claim 1 wherein the hardned plaster of Paris mold is shaped subsequent to setting.
 4. A method as in claim 1 wherein the metal oxide layer is sprayed to be approximately in the range of 0.002 to 0.005 inches thick.
 5. A composite slip cast mold structure having a smooth porous surface in the mold to receive the slip and a porous body to absorb the water from the slip comprising a flame sprayed porous refractory metal oxide layer that is initially formed to shape by being sprayed as molten droplets and frozen in situ on a metal or ceramic form conditioned to permit the form to be later separated from the flame sprayed material and having the shape of the slip casting to be produced and a mass of plaster of Paris material integrally bonded to said solidified layer; said metal oxide layer having a smooth exposed slip receiving surface and a rougher interface surface that is bonded to said plaster of Paris material; said smooth surface having approximately the same degree of finish as the surface of the mold onto which the molten metal oxide was sprayed; said rougher interface surface being the exposed surface of the layer resulting from flame spraying and freezing the metal oxide in situ on a mold; and said metal oxide layer being hard, wear resistant and porous.
 6. A mold structure as in claim 5 wherein the slip receiving layer is alumina.
 7. a mold structure as in claim 5 wherein the slip receiving layer is approximately 0.002 to 0.005 inches thick.
 8. A mold structure as in claim 6 wherein the alumina layer is about 10 percent open and interconnected pores.
 9. A mold structure as in claim 8 wherein the porous alumina layer is approximately 0.004 inches thick. 